The invention relates to the field of the extrusion of plastic or viscoelastic materials and, in particular, to the field of the extrusion of rubbery materials.
Industrial use of these materials, before they are shaped by injection moulding or by feeding them through a die, requires these materials to be homogenized as fully as possible both in terms of temperature and in terms of viscosity so as to gain control over the mechanical properties of the end-product.
Conventional extrusion methods for ensuring these functionalities comprise a screw in which the shape of the flight is designed to cause the material to circulate, while breaking up and recombining the streams. They may comprise a filter screen pack positioned downstream of the stream, so as to remove undesirable foreign substances.
These methods are generally very penalizing in terms of the flow rate and temperature performance of the extrusion means, because of the high pressure drops they cause. This penalty is all the worse if a filter system is added, because the build-up of substances may block the filters, making operation more difficult.
Already known in the prior art is an extrusion device comprising two or more kneading and conveying screws positioned coaxially and concentrically with respect to one another in the same body, in which the flight trough of a larger-diameter screw has orifices allowing the space swept by the flight of the larger-diameter screw to communicate with the space swept by the flight of the immediately adjacent smaller-diameter screw. This type of device is described by way of example in patent FR 1 270 314 or alternatively in patent FR 2 008 656 (corresponding to Schippers U.S. Pat. No. 3,583,684).
Such devices are able to achieve a certain level of homogenization but are restricted in this action in as much as the material passes directly from the space swept by the flight of the larger-diameter screw into the space swept by the flight of the immediately adjacent smaller-diameter screw. Furthermore, because of this type of layout, the screws have to turn at different speeds from one another so as to provide the work necessary for transferring the material. This leads to complex mechanical layouts, particularly when it is desirable to produce a device comprising more than two concentric screws.
Also known, from publications Meskat et al. U.S. Pat. No. 2,948,922 or SU 1 498 623, are a device and a method in which a larger diameter screw and a smaller diameter screw are arranged coaxially and telescopingly. The flight trough of the larger diameter screw has orifices which open into a space disposed radially between the telescoping screw portions. A fixed cylindrical apertured tube is interposed between a space swept by the flight of the smaller-diameter screw and said space into which the orifices open. However, these devices have the disadvantage of not easily allowing substances larger in size than the orifices made in the cylindrical tube to be removed without the need to shut the device down in order to carry out an operation of emptying and unclogging the filter.